1. Field of the Invention
Embodiments relate to hard disk drives, and specifically to a suspension assembly for supporting the moveable head of a hard disk drive.
2. Related Art
Disk drive storage systems typically include a plurality of magnetic storage medium disks stacked on a rotatable spindle and a corresponding number of magnetic heads that read binary digital information from and write binary digital information to the disks. The heads are typically supported by an actuator assembly. The actuator assembly normally includes a voice coil motor that causes pivotal motion of an actuator arm about an axis. A load beam is attached to the actuator arm and extends outward from the actuator arm over a disk surface. The load beam typically includes a base section that is affixed to the actuator arm, a hinge region integral with and extending from the base section, a rigid region integral with and extending from the hinge region, and a head suspension assembly affixed to a distal end of the rigid region. The head suspension assembly is comprised of a head slider containing the read and write heads, and a head suspension that uses a spring or gimbal structure to support the head slider in a manner that allows the head slider to move in pitch and roll directions relative to the disk surface. The head slider has an aerodynamic design that causes the head slider to fly over the surface of the rotating disk while supported on an air bearing generated by the rotating disk. A microactuator may be provided between the load beam and the head suspension assembly to provide additional control of the position of the head over the disk.
The position of the head over the disk is controlled using a servo control feedback loop. The control loop receives a reference signal that indicates the desired position for the head over the disk. This signal is combined with a position signal generated by the head that indicates the actual position of the head over the disk. A position error signal that indicates the distance between the desired position and the actual position is generated. Based on the position error signal, a controller in the control loop issues control signals to the voice coil motor and microactuator to move the head toward the desired position.
Load beams and head suspension assemblies can experience deformations that increase track misalignment range (TMR). One source of deformations is the load beam's tendency to bend and twist in a number of different modes, known as resonant frequencies, when driven back and forth at certain rates. The three primary modes are known as the sway, first torsion, and second torsion modes. The sway mode is a lateral bending mode in which the suspension bends in a transverse direction along its entire length. The first and second torsion modes are twisting modes during which the suspension twists about a rotational axis which extends along the length of the suspension. Another source of deformations is windage, which is movement of the load beam or head suspension assembly due to airflow forces. Deformations can also be caused by movement of the microactuator.
To prevent off-track head motion caused by these forces, the prior art devices have placed sensors on the actuator assembly to sense deformations and supply signals to the control loop. U.S. Pat. No. 5,862,015 shows a sensor provided on a load beam that detects strain in the actuator assembly caused by movement of the head relative to the actuator arm. U.S. Pat. No. 6,771,454 describes a control system that minimizes off-track motion due to resonant modes by sensing movement with a sensor provided on a load beam. The sensor can be one of many types, such as a pressure sensor, capacitive plate position sensor, Micro Electro Mechanical System (MEMS) accelerometer, MEMS-based piezo-resistive sensor, and polyvinylidene fluoride (PVDF) film sensor.
As performance and resonant frequency requirements have become more stringent, load beam designs have become more complex, as have the methods of forming the load beams. U.S. Pat. No. 6,938,326, describes a method of forming a load beam in which the hinge region of the load beam is comprised of two arms separated by an opening, and the spring rate of the hinge region is controlled by etching to adjust the thickness of its arms relative to the remainder of the load beam. The rigid section of the load beam is provided with bent edge portions, referred to as rails, that maintain the rigidity of the structure.